Introduction to the Landforms and Geology of Japan

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Hokkaido

Hokkaido, the northernmost main island, was formed by which the Kuril Arc collided with the Northeast Japan Arc at the central part of Hokkaido after the Middle Miocene. Therefore, Hokkaido is divided into three parts: the eastern region (part of the Northeast Japan Arc), the central region (an arc-arc collision zone), and the western region (part of the Kuril Arc). Two island arcs meet each other in the central region but the region is not a plate boundary zone. It is thought that there was a plate boundary in the central region and it shifted to the margin of the Sea of Japan in the end of the Tertiary or the Early Quaternary. The volcanic front runs nearly east-west from the eastern region through the central region and turns to the south in the western region. This front line is parallel to the Kuril Trench and the Japan Trench.

Hokkaido has broad gentle landforms and less mountainous areas compared to Honshu, Shikoku, and Kyushu. The mountains in Hokkaido generally have gentle slopes in the northern part and steep slopes in the southern part.

For the basement rocks (geotectonic subdivisions), see also “Outline of landforms and geology of Japan”.

Eastern region

In the eastern region, northeastward trending uplift areas with volcanoes are arranged in echelon toward the Kuril Islands, including the Akan volcanoes, the Shiretoko Peninsula, the Kunasir Island, and the Etorofu Island. The en echelon arrangement of the uplift areas is attributed to the oblique subduction of the Pacific Plate at the Kuril Trench. The Konsen Plateau, the Kushiro Plain, and the Tokachi Plain spread in the Kuril outer arc, on the south of the volcanic area. Massive volcanic products and clastic material produced by erosion transferred from the volcanoes onto the Konsen Plateau and the Kushiro Plain to form plateaus and hills. This sediment covers Cretaceous and Paleogene rocks. The Shiranuka Hills situated between the Kushiro Plain and the Tokachi Plain are a low relief uplift area. Mountains in the central part of the Shiranuka Hills comprise Cretaceous and Paleogene rocks, and they are surrounded by hills consisting of Neogene and Quaternary formations. Pliocene-Quaternary formations are distributed in the Tokachi Plain.

The Kitami Mountains are placed in the Kuril inner arc, to the east of the Teshio Mountains. This mountain range is the stablest area in Japan, in which active faulting and earthquakes rarely occur. The Kitami Mountains consist mainly of Cretaceous-Paleogene basement rocks and volcanic rocks that erupted in the Miocene or later and covered the basement. The volcanic rocks formed lava plateaus. The Cretaceous-Paleogene rocks in the Hidaka Supergroup consist of turbidite sandstone and mudstone, and mélange including chert, limestone and greenstone (accretionary complex).

The Kussharo volcano in the eastern region is accompanied with the largest caldera (26 × 20 km) in Japan.

Central region

The Teshio Mountains and the Yubari Mountains are located in the central region, both of which have north-south main ridges. A depression zone is placed between the Teshio-Yubari Mountains and the Kitami-Yubari Mountains. The Teshio Mountains consist of Cretaceous-Tertiary rocks, produced by folding with faults. The Yubari Mountains have Jurassic-Cretaceous rocks (Sorachi Group consisting of greenstone [including basaltic pillow lava, hyaloclastite, and diabase], chert, micrite limestone, and sandstone with felsic tuff) and serpentinite in and around the main ridge. Cretaceous forearc sediments (Yezo Supergroup) are distributed around the Jurassic-Cretaceous rocks and Paleogene sediments with interbedded coal seams are found on the west of the Cretaceous sediments. These mountains were upheaved by the collision of the two island arcs.

The Hidaka Mountains in the western margin of the Kuril outer arc are situated to the southeast of the Yubari Mountains. This mountain range was also uplifted in association with the colliding Kuril Arc. In the Hidaka Mountains, the Hidaka metamorphic rocks, the Cretaceous-Paleogene system, and the Cretaceous system are distributed in the central part, eastern part, and western part, respectively (Figure). The crust of the island arc and the upper mantle were thrusted up on the metamorphosed oceanic crust along the Hidaka thrust fault. The Hidaka metamorphic zone, therefore, is regarded as the exposed cross section of the island arc crust. High pressure type metamorphic rock zone (Kamuikotan Belt) is found on the west of the Hidaka metamorphic zone.

The Daisetsu and Tokachi volcanic area (Ishikari Mountains) between the Hidaka Mountains and the Kitami Mountains are the highest elevation area in Hokkaido (the highest peak is 2290 m high).

Western region

The Ishikari Plain is an alluvial lowland to the west of the Yubari Mountains. The Oshima Peninsula is located on the west of the Ishikari Plain, which is part of the Northeast Japan Arc. Mountains, volcanoes, and plains in the peninsula are distributed complicatedly. This irregular arrangement is a different feature from the zonal arrangements of landforms in the central and eastern regions. A Jurassic accretionary complex intruded by Cretaceous granite is found in this region, which is uncoformably covered with Neogene-Quaternary volcanic rocks and sediments.

Some volcanoes such as the Shikotsu volcano and the Toya volcano in this region have large calderas and pyroclastic plateaus.

References


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